Various impurities are contained in copper ore, and one of the impurities is arsenic (As). While most of arsenic (As) is evaporated and separated by a high temperature during a dry process of the copper smelting, it is partially entrained in crude copper and carried into the subsequent copper electrorefining process.
Part of As contained in the crude copper (copper anode) is eluted into electrolytic solution, and the remaining undissolved As is mixed in anode slime deposited on the bottom of an electrolytic tank. In addition, since the amount of copper eluted from the anode is generally larger than that of copper deposited on the cathode, the copper concentration will increase gradually in the electrolytic solution. Therefore, a part of the electrolytic solution is extracted into another electrolytic tank to control the quality of the electrolytic solution. Decoppering electrolysis is conducted to the extracted electrolytic solution to separate and recover Cu and impurities such as As by precipitating them on the cathode or depositing them on the bottom of the electrolysis bath. In this technical field, the substances deposited on the bottom of the electrolytic tank and those precipitated on the cathode are called as “electrolytically precipitated copper”.
Electrolytically precipitated copper is typically fed back to the copper smelting process. For this purpose, impurities such as As should be desirably removed from the electrolytically precipitated copper. In addition, As in itself can be used as valuable material. Accordingly, there is a need for a technology to separate and recover As from electrolytically precipitated copper with high grade. In this regard, Japanese unexamined patent publication No. 6-279879 discloses a method comprising adding electrolytically precipitated copper to sulfuric acid solution so that leaching reaction occurs and then separating sulfuric acid-leached solution containing As and Cu from leaching residue containing Bi and Sb. The publication states in the examples section that electrolytically precipitated copper was added to 100 g/L sulfuric acid solution (the pH is speculated to be about −0.3) to cause the sulfuric acid leaching.
Furthermore, it has been known that crystallization of scorodite (FeAsO4.2H2O), which is an iron-arsenic compound, is effective in fixing arsenic. Crystalline scorodite is chemically stable, and suitable for long-term storage. On the other hand, amorphous scorodite is not stable, therefore not suitable for long-term storage.
For example, Japanese patent No. 3,756,687 discloses a method for removing and fixing arsenic from solution containing arsenic and non-ferrous metal component including copper and/or zinc. The method comprises a first step of adding iron (II) solution and/or iron (III) solution to the arsenic-containing solution for reaction at 120° C. or higher to form scorodite having stable crystallinity as an iron-arsenic compound and then recovering scorodite containing non-ferrous metal components such as copper from the arsenic-containing solution by solid-liquid separation, and a second step of adding water to the obtained scorodite containing non-ferrous metal components such as copper for repulping so that the non-ferrous metal components such as copper is dissolved into the solution and separated from the scorodite.
The patent states that in this way arsenic can be removed and fixed as stable crystalline scorodite without losing valuable metals such as copper.
Furthermore, Japanese unexamined patent publication No. 2005-161123 discloses a method for removing arsenic from soot containing arsenic. The method comprises a leaching step for leaching arsenic from the arsenic-containing soot with acid solution, a precipitation reaction step for precipitating amorphous iron arsenate by mixing iron ion-containing acidic aqueous solution in the leached solution, and a crystallization step for crystallizing the amorphous iron arsenate by heating the mixed solution. The crystallized iron arsenate is removed by filtering the mixed solution.
The publication states that the method can remove arsenic from soot very easily since it needs no additional processes, such as pH adjustment, once the iron ion-containing acidic aqueous solution is mixed in the leached solution.
Furthermore, it states in the embodiment section that sulfuric acid solution (concentration is 0.2 mol/L, i.e., pH is about 0.4) was used to leach arsenic from soot, and the pH of both the leached solution and the iron ion-containing acidic aqueous solution (ferric sulfate) was 1.0 to 1.5.